Entrance pupil

The entrance pupil is the image of an instrument's aperture stop as viewed through the optics preceding that stop from the object side. It is the apparent opening that limits the accepted bundle from an on-axis object point; it need not coincide with a physical hole.

If no optics precede the aperture stop, the two coincide. If an optical lens lies in front of a camera iris, the entrance pupil can have a different apparent size and position from the iris. This distinction matters when comparing light collection or placing an instrument's rotation point to reduce parallax between photographs.